Data transmission systems of current use include now a keyboard, i.e., an input point data-entry unit wherein only one input point can be activated at a time, at any given time.
Such an entry unit system raises a problem in connection to data transmission. Such a transmission can be achieved in an analogical way. Such is the case in telephone keyboards wherein the activation of a key entails the transmission of a combination of several signals of different frequencies, each key being associated with a particular combination. Such an analogical transmission system is both costly and cumbersome while the improvements achieved in the integrated circuit technology are rapidly progressing to miniaturization together with a considerable drop of the prices.
Therefore, the use of digital data transmission is widely spread and, more particularly, is used in local transmission systems wherein an operator keys in data on his typewriter type keyboard terminal, which data are transmitted to a processor for tape-recording or printing out.
At present, the technique of wide use consists in transmitting the digital data in parallel form. This requires the use of a cable connecting the input terminal to the processor, which has as many wires as there are input points. It is obvious that this type of multiwire cable is costly because, from amongst other things, it is necessary, for instance, to have multipin connectors, which is not always of practical use.
Because of the above-mentioned drawbacks, it is, therefore, preferred to proceed to a serial transmission of the digital data coming from the terminal or entry unit. Such a serial transmission can be carried out either synchronously or asynchronously. The asynchronous transmission process consists in transmitting each data character that corresponds to the activation of an input point over the line, in an independent way, by means of a code beginning with a start bit and ending with one or two stop bits. On the reception side, synchronization is achieved upon detection of the start and stop bits. Unfortunately, when desynchronization occurs, this cannot be detected rapidly, thereby entailing an erroneous data reception.
In the so-called synchronous transmission process, data characters are within character frames having special characters at regular intervals, which are used for synchronization as start and, possibly, stop flags for further transmission of bit sequences or packets.
A system using such special characters is described in U.S. Pat. No. 3,335,404 in which the occurence of a particular event at a remote station modifies a chain code generated by a chain code generator, by inverting one or several bits of the chain code. The modified or unmodified chain code is transmitted to an intermediate station which transmits to the receiving station complete cycles of the modified chain codes received from the remote station together with an indication of which remote station the modified chain code has been received from.